Coating materials, e.g. photoresists, are typically applied to a semiconductor wafer by flowing a coating liquid onto the wafer while spinning the wafer. A centrifugal flow component is imparted to the liquid causing it to spread evenly over the wafer surface. Excess material is ejected from the wafer periphery. The objective is to obtain a uniform coating over the entire surface.
These coating processes often result in an edge bead that develops around the periphery of the wafer due to the meniscus that characteristically forms in the liquid at the edge of the wafer. Wafer fabrication problems introduced by this edge bead are described in our earlier patent application Ser. No. 232,739, now abandoned. Techniques for removing edge beads are described in this earlier application and also in U.S. Pat. No. 4,113,492 issued Sept. 12, 1978 to Sato and Fuji.
VLSI circuits can only be successful if the total defect density at all manufacturing steps is held under strict control. Manufacturing experience with the edge bead removal techniques just referenced was unsatisfactory in this respect, even though the induced defect density was very low. The configurations described by Sato and Fuji are not even remotely satisfactory in this respect.